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From: "hjl.tools at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/107172] [13 Regression] wrong code with "-O1 -ftree-vrp"
 on x86_64-linux-gnu since r13-1268-g8c99e307b20c502e
Date: Thu, 13 Oct 2022 22:23:44 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107172

--- Comment #31 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Segher Boessenkool from comment #29)
> (In reply to Hongtao.liu from comment #23)
> > looking at i386.c put_condition_code used by *setcc_qi, it looks like (=
EQ
> > (reg:CCCmode FLAG_REG) (const_int 0)) means get carry flag.
> > Not (LTU: (REG:CCCmode FLAGS_REG) (const_int 0)).
> > Now I got more confused.
>=20
> (eq (reg:CCC 17) (const_int 0))  means that the comparison that did set r=
eg
> 17
> returned "equal".
>=20

Isn't the meaning of MODE_CC comparison only known to the backend? On x86, =
EQ
in

  (set (pc) (if_then_else
               (eq (reg:CCO FLAGS_REG) (const_int 0))
               (label_ref (match_operand 3))
               (pc)))]

doesn't evaluate to true when (reg:CCO FLAGS_REG) is 0.  It is true when
(reg:CCO FLAGS_REG) is set.=